I have a black dot in my eye. Is this serious?

Floaters. Floaters are thickenings in the vitreous jelly inside the eye. They are common with age, myopia, inflammation, trauma, and other issues. They are benign and do not damage your vision. Rarely, they can cause retinal tears which may require laser treatment. Otherwise, no treatment is needed. In very rare cases, the floaters can become dense enough to block vision and may need surgical removal.
See eye doctor. Would need to know if you see this on your eye when you look in the mirror or when you look at something, you see a black dot on the image you are looking at. If you see it in the mirror, need to make sure it is a freckle and not a malignancy. If you see it when you look at images, you need to make sure there is not a problem with the retina, the jelly part of the eye, or the optic nerve.